The retinal pigment epithelial (RPE) cell plays a basic role in maintaining the structural and physiological integrity of the neural retina. We have isolated and propagated RPE cells in vitro and have developed monoclonal antibodies directed against human RPE cells. The RPE epitope is a 67 kDa protein that is closely associated with the microsomal membrane. A cDNA clone has been isolated that codes for a protein that does not match any other sequences in the databases. We are using these techniques and reagents to evaluate molecular, biochemical, and bioloical properties of the RPE cells. Cytokines, such as interferon (IFN)-gamma and interleukin-2 (IL-2), are a group of specialized hormone-like proteins that exert profound influences on cellular development and on a variety of cellular functions. This project has concentrated on studying the ways in which cytokines interact with cells of the immune system and with cells in the ocular microenvironment. We have shown that IFN-gamma and IL-2 are found within the inflamed eye in association with T-cell infiltration and major histocompatibility complex (MHC) class II antigen expression on infiltrating cells and on RPE cells. Furthermore, IFN-gamma activated RPE cells can process and present antigens to helper T lymphocytes. These studies indicate that cytokine-mediated activation of RPE cells may be a basic component of ocular immunity and an important aspect of RPE cell transplantation. IL-1, tumor necrosis factor (TNF), and IFN-gamma are potent proinflammatory cytokines that can also activate RPE cells to produce IL-6 and intracellular adhesion molecule-1 (ICAM-1) mRNA and proteins. Moreover, these cytokines induce nitric oxide (NO) in RPE cells. NO is cytotoxic and may play an important role in inflammatory and neurodegenerative diseases. Cytokines frequently can interacte with each other in synergistic or antagonisitc processes that can alter cellular function. An understanding of these interactions may provide rationale to modify cellular responses. Using this approach, we found that proinflammatory cytokine (IL-1, TNF, IFN-gamma) induction of NO in RPE cells can be inhibited by transforming growth factor (TGF)-beta. Using reverse transcriptase-polymerase chain reaction (RT-PCR), we found that TGF-beta inhibits cytokine-mediated induction of NO synthetase mRNA in RPE cells. Studies are also in progress to propagate and transplant RPE cells in various animals. We have established a graft rejection model by transplanting human RPE cells into the rat retina. These studies demonstrate that both cellular and humoral aspects of the immune response are activated to reject RPE cell transplants. These studies provide the framework to evaluate cytokines and immune reactivity in RPE cell transplantation.